1. Field of the Invention
The present invention relates to an air-fuel ratio control device for an internal combustion engine, and more specifically, relates to an air-fuel ratio control device which controls the air-fuel ratio of the engine based on the outputs of air-fuel ratio sensors disposed on an exhaust gas passage upstream and downstream of a catalytic converter.
2. Description of the Related Art
Three-way reducing and oxidizing catalytic converters are commonly used in order to remove three pollutants, i.e., NO.sub.x, HC, and CO components in the exhaust gas of an internal combustion engine. Generally, the catalyst used in such converters is able to remove the these three pollutants from the exhaust gas simultaneously only when the air-fuel ratio of the exhaust gas is kept in a narrow range near the stoichiometric air-fuel ratio. Therefore, in order to reduce the emission of the exhaust gas, it is important to keep the air-fuel ratio of the exhaust gas in the region near the stoichiometric air-fuel ratio.
For this purpose, an air-fuel ratio control device for controlling the air-fuel ratio of an engine by feedback control based on an output of one air-fuel ratio sensor (such as an O.sub.2 sensor) disposed on an exhaust passage upstream of a catalytic converter, is used to maintain the air-fuel ratio of the engine in a desired range. This type of the air-fuel ratio control system is known as a single air-fuel ratio sensor system. In the single air-fuel ratio sensor system the air-fuel ratio of the exhaust gas flowing into the catalytic converter is detected by the air-fuel ratio sensor disposed on the exhaust gas passage upstream of the catalytic converter and the amount of the fuel fed to the engine is feedback controlled based on the output of the air-fuel ratio sensor in such a manner that the air-fuel ratio of the exhaust gas flowing into the catalytic converter is maintained at stoichiometric air-fuel ratio. (In this specification, the term "an air-fuel ratio of the exhaust gas" means a ratio of the total amounts of the fuel and the air which are fed to the engine and, if any, to the exhaust gas passage upstream of the catalytic converter. Further, the term "an air-fuel ratio of the engine" means an air-fuel ratio of the combustion in the combustion chamber of the engine. Therefore, the air-fuel ratio of the exhaust gas becomes the same value as the air-fuel ratio of the engine when neither a fuel nor a secondary air is fed to the exhaust gas passage upstream of the catalytic converter.
However, in some cases, the air-fuel ratio of the engine is not precisely controlled at the stoichiometric air-fuel ratio in the single air-fuel ratio sensor system.
In the single air-fuel ratio sensor system, the accuracy of the air-fuel ratio control is directly affected by individual differences in the output characteristics of the air-fuel ratio sensor. Also, the output characteristics of the air-fuel ratio sensor may change gradually due to the deterioration caused by the high temperature of the exhaust gas upstream of the catalytic converter. Further, the exhaust gases from the respective cylinders are not mixed uniformly in the exhaust passage upstream of the catalytic converter, and the output of the air-fuel ratio sensor may reflects only the air-fuel ratio of the exhaust gas from a specific cylinder of the engine, i.e., the air-fuel ratio of the engine as a whole may not be detected by the air-fuel ratio sensor upstream of the catalytic converter.
In order to compensate for the individual difference among cylinders or changes due to the deterioration of the upstream air-fuel ratio sensor, a double sensor system using two air-fuel ratio sensors has been developed (U.S. Pat. No. 4,739,614).
In the double sensor system, air-fuel ratio sensors are disposed upstream and downstream of the catalytic converter in the exhaust passage, and the air-fuel ratio control is carried out based on the output of the downstream air-fuel ratio sensor as well as the output of the upstream air-fuel ratio sensor. Since the exhaust gases from the respective cylinders of the engine are mixed uniformly on the downstream side of the catalytic converter, the air-fuel ratio sensor disposed on the downstream side of the catalytic converter is not affected by a specific cylinder. Further, since the exhaust temperature is low on the downstream side when compared with upstream side, the change in the output characteristics of the downstream air-fuel ratio sensor due to deterioration is relatively small. Therefore, in the double sensor system, the air-fuel ratio of the engine is accurately controlled by correcting the output of the upstream air-fuel ratio sensor based on the output of the downstream air-fuel ratio sensor.
Nevertheless, there is a problem in the double sensor system in the related art. In the double sensor system, there exists a delay in the response of the downstream air-fuel ratio sensor to detect a change in the air-fuel ratio of the engine.
This delay in the response of the downstream air-fuel ratio sensor is caused by an oxygen storage capacity (O.sub.2 storage capacity) of the three-way reducing and oxidizing catalyst in the catalytic converter. Usually, the three-way reducing and oxidizing catalyst is provided with a so-called O.sub.2 storage capacity, i.e., a capability of absorbing oxygen in the exhaust gas when the air-fuel ratio of the exhaust gas is lean compared with the stoichiometric air-fuel ratio, and releasing the absorbed oxygen when the air-fuel ratio of the exhaust gas is rich compared with the stoichiometric air-fuel ratio. Due to this O.sub.2 storage capacity, the atmosphere in the catalytic converter is maintained at near the stoichiometric air-fuel ratio even when the air-fuel ratio of the exhaust gas deviates from the stoichiometric air-fuel ratio for a short period. Though the O.sub.2 storage capacity is necessary to utilize the ability of the catalyst to a maximum degree, the response of the downstream air-fuel ratio sensor to the change in the air-fuel ratio of the engine becomes poor due to the absorbing and releasing action of the oxygen by the catalyst.
For example, when the air-fuel ratio of the exhaust gas flowing into the catalytic converter changes from a lean side air-fuel ratio to a rich side air-fuel ratio compared with the stoichiometric air-fuel ratio, the air-fuel ratio of the exhaust gas flowing out from the catalytic converter does not change immediately since the oxygen absorbed in the catalyst is released when the air-fuel ratio of the exhaust gas flowing into the catalyst becomes rich. In this case, the air-fuel ratio of the exhaust gas flowing out from the catalyst changes to rich compared with the stoichiometric air-fuel ratio, only after the oxygen in the catalyst is completely released, i.e., the change in the air-fuel ratio of the exhaust gas downstream of the catalyst is delayed compared with the change in the air-fuel ratio of the exhaust gas upstream of the catalytic converter.
Therefore, when the air-fuel ratio of the engine starts to deviate to the rich air-fuel ratio side from the stoichiometric air-fuel ratio for some reason during air-fuel ratio control, the air-fuel ratio of the engine deviates largely to a rich air-fuel ratio side before the change in the air-fuel ratio of the engine is detected by the downstream air-fuel ratio sensor. A similar delay will occur when the air-fuel ratio of the engine deviates to the lean air-fuel ratio side. Because of this delay in the detection of the air-fuel ratio of the engine by the downstream air-fuel ratio sensor, it is difficult to compensate the output of the upstream air-fuel ratio sensor accurately based on the output of the downstream air-fuel ratio sensor.
To compensate for the delay in the response of the downstream air-fuel ratio sensor, Japanese Unexamined Patent Publication (Kokai) No. 63-195351 proposes a double sensor system which changes the values of the factors used in the feedback control by the upstream air-fuel ratio sensor in accordance with the magnitude of the deviation of the air-fuel ratio detected by the downstream air-fuel ratio sensor from the stoichiometric air-fuel ratio. Namely, in the double sensor system in the above publication, when the deviation of the air-fuel ratio at downstream of the catalytic converter becomes larger, the factors used in the feedback control by the upstream air-fuel ratio sensor are largely changed so that the air-fuel ratio of the engine converges to the stoichiometric air-fuel ratio in short time.
However, in the double sensor system disclosed in Japanese Unexamined Patent Publication (Kokai) No. 63-195351, the factors used in the feedback control are determined in accordance with the output of the downstream air-fuel ratio sensor which has delayed response due to the O.sub.2 storage capacity of the catalyst. This means that, in the above double sensor system, when the output of the downstream air-fuel ratio sensor is the same, the factors used in the feedback control are set at the same values regardless of whether the air-fuel ratio of the engine starts to deviate to the rich air-fuel ratio or the lean air-fuel ratio. Therefore, accurate air-fuel ratio control which reflects the tendency of the change in the air-fuel ratio of the engine cannot be achieved by the above double sensor system.